fix: close review findings on the velocity-curve deck and bipolar curves

Cancels the curve-node drag whenever the popup closes so Esc mid-drag can't alias the amp curve; generalizes CurveTarget routing to one switch; fixes stale/overstated comments; clamps a pre-v12 depth fold; adds deck-inertness and filter-fold test coverage.

src/shell/panel/draw_kit.h

@@ -87,6 +87,11 @@ void fillSurface(LICE_IBitmap* bmp, const KitBox& box, Role role, InteractionSta
 void drawButton(LICE_IBitmap* bmp, const KitButtonBox& button, const char* label,
                 InteractionState state, bool warn);
 
+// THE ink role drawButton's own label draws in, for `state` (see draw_kit.cpp). A non-text
+// button mark (e.g. a drawn glyph, not text()) that needs to sit legibly on a drawButton
+// surface should call this rather than re-deriving the rule.
+Role buttonLabelRole(InteractionState state);
+
 // A horizontal slider: track groove, accent-filled portion up to the handle, and the
 // handle itself. `geom` is the pure SliderGeometry the caller computed.
 void drawSlider(LICE_IBitmap* bmp, const SliderGeometry& geom, InteractionState state);